The present invention relates to an optical disk player wherein a read output signal is obtained by detecting a laser beam emitted to be incident on a data-recorded optical disk and reflected therefrom, and the recorded data are reproduced on the basis of such read output signal.
In optical disks used widely as data recording media, there exist some different types including, for example, one where recorded data are merely readable for general users but new data recording is impossible, and another where recorded data are readable and further new data recording is also possible. However, in any type of such optical disks, an operation of reading the recorded data is performed under conditions that a light beam is emitted to be incident on a data recording plane of the disk being driven at a predetermined rotation speed, and the data recorded portion is scanned by the light beam.
Various types of optical disk players are proposed currently as apparatus for reading recorded data from such an optical disk and obtaining reproduced data. An optical disk player comprises, as its fundamental component units, a disk driver for rotating an optical disk at a predetermined speed, an optical head for causing a light beam to be incident on a data recording plane of the optical disk being rotated, and then detecting therefrom a reflected light beam or a transmitted light beam to obtain a read output signal as a detection output, a read signal processor for obtaining a read data signal by processing the read output signal from the optical head, and a data reproducer for reproducing the data on the basis of the read data signal obtained from the read signal processor. Normally the optical disk player further has a tracking servo control system for properly maintaining the incidence position of the light beam emitted from the optical head to be incident on the data recording plane of the optical disk, and a focus servo control system for properly maintaining the focus state at the incidence position of the light beam emitted from the optical head to be incident on the data recording plane of the optical disk.
In such optical disk player, the light beam emitted to be incident on the optical disk for detecting the data recorded thereon is composed generally of a laser beam. More specifically, a laser light source consisting of a semiconductor laser device to perform single longitudinal-mode light emission (single wavelength light emission) is incorporated in the optical head to form a laser beam which is composed of the laser light emitted from the laser light source, and the laser beam is caused to be incident on the optical disk.
The laser beam incident on the optical disk is reflected therefrom while the intensity thereof is modulated in accordance with the data recorded on the disk, and then the reflected laser beam is guided to a light receiver incorporated in the optical head. Subsequently in response to the reflected laser beam from the optical disk, the light receiver generates a read output signal proportional to the intensity-modulated state of the reflected laser beam and then sends the output signal to a read signal processor.
In the semiconductor laser device which constitutes a laser light source in the optical disk player as mentioned and performs single longitudinal-mode light emission, there occurs a transition of the light emission mode due to the temperature fluctuation induced during the operation or some other influence of a return beam and so forth caused by reflection of the emitted laser beam, and consequently the wavelength and the power of the emitted laser beam are varied. Such wavelength and power variation in the laser beam emitted from the laser light source brings about an undesired situation where the read data signal, which is obtained from the read signal processor having received the read output signal from the light receiver, includes some noise component derived from the laser beam variation (hereinafter referred to as laser noise component).
The laser noise component included in the read data signal lowers the S/N (signal-to-noise ratio) in the read data signal to consequently exert harmful influence on the reproduced data outputted from the data reproducer which serves to reproduce the data on the basis of the read data signal. More concretely, such harmful influence on the reproduced data deteriorates the error rate in the reproduced data obtained in digital form, or lowers the S/N in the analog audio or video signal obtained as reproduced data.
In order to reduce the wavelength and power variation in the laser beam emitted from the semiconductor laser device constituting the laser light source, it has been customary heretofore to adopt some techniques of executing high frequency control with regard to the semiconductor laser device which performs single longitudinal-mode light emission, or employing a different laser device which performs multiple longitudinal-mode light emission. High frequency control for the semiconductor laser device of single longitudinal-mode light emission type is executed by operating the same through on/off driving with a high frequency signal of several hundred MHz or so. A semiconductor laser device has such characteristic as to perform multiple longitudinal-mode light emission when its operation is in an instantaneous on-state, so that multiple longitudinal-mode light emission seems to be continuous falsely if fast on/off driving is repeated at a high frequency. Meanwhile, in using another device of multiple longitudinal-mode light emission type, transition of the light emission mode is diminished to consequently suppress the wavelength and power variation in the emitted laser beam. Therefore, due to such high frequency control for a semiconductor laser device of single longitudinal-mode light emission type, it becomes possible to reduce the wavelength and power variation in the laser beam emitted therefrom.
Meanwhile the semiconductor laser device of multiple longitudinal-mode light emission type used as a laser light source is structurally so contrived that its state of multiple longitudinal-mode light emission is kept continuous merely by DC driving. In such semiconductor laser device where a multiple longitudinal-mode light emission state thereof is kept continuous, the transition of the light emission mode resulting from any temperature fluctuation or return light during the operation is suppressed to consequently reduce the wavelength and power variation in the emitted laser beam.
However, when high frequency control is to be performed for the semiconductor laser device of single longitudinal-mode light emission type, it is necessary in the circuit configuration to incorporate a high frequency oscillator and a high frequency driver. Although the wavelength and power variation in the laser beam emitted from the semiconductor laser device is thus reduced with certainty, such variation is still left partly to some extent, and it is difficult to achieve complete suppression of the laser noise component included in the read data signal.
Meanwhile in the case of using a laser device of multiple longitudinal-mode light emission type, there exist some difficulties in manufacture of a superior device which is capable of emitting a laser beam with a relatively greater power in comparison with a semiconductor laser device of single longitudinal-mode light emission type, and another disadvantage is unavoidable in relation to a short service life. Further, even in the use of a multiple longitudinal-mode light emitting laser device, the wavelength and power variation in the laser beam emitted from the semiconductor laser device is still left partly to some extent although being reducible with certainty, and any wavelength and power error of the laser beam derived from the individual deviation of the multiple longitudinal-mode light emitting laser device is not corrected, so that the laser noise component included in the read data signal fails to be completely suppressed.